This invention relates to a surface acoustic wave device, and more particularly to a composition for a surface acoustic wave device.
Surface acoustic wave devices are obtained by constructing an interdigital electrode on a piezoelectric base. The width of this electrode is proportional to the wavelength (.lambda.) of the surface acoustic wave. This wavelength (.lambda.) is determined by the relation .lambda.=v/f, where v is the propagation velocity of the surface acoustic wave (sound) and f is the frequency of the surface acoustic wave. Therefore, if the frequency (f) is high, the wavelength (.lambda.) is short, hence the electrode size should be small, which leads to a decrease in the yield rate in the production of such devices. Consequently, bases for surface acoustic wave devices to be used in a high frequency region are desired to propagate sound as rapidly as possible.
Thus far, various kinds of glass, such as fused quartz having a piezoelectric zinc oxide coating layer, have been used as the bases for surface acoustic wave devices. However, in these cases, the propagation velocity of sound is quite low and moreover the effective electromechanical coupling coefficient K.sup.2, a fundamental factor decisive of the performance of the devices, cannot have a satisfactory good value.
For example, for a fused quartz base having a zinc oxide layer thereon, the propagation velocity of sound (v) is about 2.7 km/s and the coefficient K.sup.2 is about 3%.
A well known material with a high coupling coefficient (K.sup.2) value is crystalline LiNbO.sub.3. The K.sup.2 value of crystalline LiNbO.sub.3 is at most 5-6% depending on the crystalline orientation. The propagation velocity of sound is at most 4 km/s. However, the frequency temperature coefficient TC(f) (.ident.(1/f) (.differential.f/.differential.T), where f is a frequency of the surface acoustic wave and T is a temperature), a factor decisive of the performance of a surface acoustic wave device, is -77 ppm/.degree.C. and therefore temperature stability is disadvantageously lacking.
On the other hand, as a material propagating sound rapidly, there may be employed a piezoelectric material such as aluminum nitride. For aluminum nitride, the propagation velocity of sound is as high as 6 km/s, and accordingly the corresponding size of the interdigital electrode is relatively large and such electrode can be processed easily. However, formation of an aluminum nitride film is not easy. As another example of such a material, a layered substrate composed of a monocrystalline .alpha.-alumina base and a monocrystalline layer of piezoelectric zinc oxide directly and epitaxially grown thereon has thus far been studied. However, for producing the monocrystalline layer, it was necessary to raise the temperature of the base to 500.degree. C. or above, and this presented difficulties in production control and other problems. At temperature below 500.degree. C., the crystallinity was inferior. Moreover, the rate of growth of the crystalline layer is slow and at most 100 A/min, and therefore the production disadvantageously requires much time.